1. Field of the Invention
This invention relates to off-shore well drilling and related equipment and, more particularly, to prefabricated drilling and production and other platforms, and prefabricated tripod and monopod support structures for maintaining the platforms at a desired elevation above the water, the platforms being buoyant and maneuverable into position relative to the supports and being provided with pre-set-up jacking means immediately operable to raise the platforms on the supports to the desired height above the water after the platforms have been moved into place.
2. Description of the Prior Art
Prefabricated drilling and production platforms and supports therefor, and jacking mechanisms for raising the platforms on the supports have been devised heretofore, but many of these are suitable only for use in shallow and relatively calm waters. Some of such prior platforms have utilized jacking mechanisms including racks and pinions with the racks attached to square spuds, but these have the disadvantage that the spuds must be pre-mounted in openings in the platform and the racks indexed with pinions mounted on the platform, so that proper engagement can be effected. Such structure is shown in Bulkley et al U.S. Pat. No. 2,308,743. Racks and pinions when applied to caissons have the further disadvantage that pressure loads are applied to local areas, which may cause collapse, or require excessive and undesirable internal bracing of the caissons. The latter is particularly objectionable when conductor pipes are to be located in the caissons.
Other prior jacking devices include annular rubber tubelike elements carried by the barge or platform and surrounding the caissons. A jack of this type is disclosed in Suderow U.S. Pat. No. 2,948,119. The rubber elements must be inflated by air to grip the caissons, and manipulated to elevate the barge on the caissons. Such devices are objectionable because the rubber elements are subject to rapid wear, blow out, require substantial maintenance and frequent replacement. Furthermore, they have a tendency to slip relative to the caissons, particularly if the exterior surface of the caissons is contaminated by oil, algae, or mud. Such devices are subject to failure by allowing the barge to drop back onto the water whenever the air supply fails, as by the rupture of an air hose.
Still another jacking device, known as a cable jack, includes criss-crossed cables that are operable for positively gripping the exterior of the caissons and raising the barge on the caissons. Such jack is disclosed in my prior U.S. Pat. No. 2,858,105. The cable jacks are much more satisfactory than the rack and pinion type jack and inflatable rubber tube device. However, all of the foregoing types of jacking devices have the disadvantage that they are slow in operation and take considerable time to "set up" and to be operated to effect elevation of the barge to the desired working height. Such slow-operating jacking devices are unsuitable for use in rough waters, and particularly in areas subject to abnormally high changes in tide levels, say, 20 to 30 feet, or to high waves because of the potential damage to either the caissons or the barge, or both, during the period that the barge is subject to movement by wave action. The hazards, of course, are less after the barge has been raised above wave level effect, but with slow-operating jacking devices it may take several hours' time to do this.
Another prior jacking apparatus, shown in Nixon U.S. Pat. No. 2,833,118, embodies the use of four sets of cables on each caisson, the cables being under tension and fastened at their opposite ends to the exterior of the caisson. A wedge type jacking mechanism is associated with each set of cables and is fixed to the barge or platform. The caissons are pre-positioned in openings in the barge. The jacking mechanisms are designed to lower the caissons to the bottom and then elevate the barge. They can be used later to raise the caissons to enable the barge to be moved to another site. Such device has the objections that the cables become worn and frayed with repeated use; the caissons must be mounted in the platform before it is towed to the site or else much time is wasted in setting them up at the site; the caissons rise and fall with the barge in response to wave action; and it is possible for the barge to lift the caissons off the bottom at any time during the jacking operation until the barge is raised high enough to be clear of the waves. This makes it difficult to locate the barge at an exact point and could result in damage to the barge and/or caissons and injury to personnel.
A prefabricated platform support structure, including a tripod that is towed to the drilling site and sunk, is not new per se. A structure of this type is disclosed in the patent to Kuss et al, U.S. Pat. No. 2,586,966. However, the patentees contemplate building a platform on the caissons after the support has been anchored to the bed of the ocean. Such procedure is objectionable because it is expensive, slow, impractical, and very hazardous. Considerable auxiliary equipment is required in the way of tugs and lighters to bring the platform components to the site, and derrick barges are required for use in erecting the components on the caissons. Obviously, no erection work can be done in rough seas, at which time all personnel and rented equipment remains idle at tremendous costs.
A prefabricated platform with three caisson wells has heretofore been used with a prefabricated tripod support in constructing a structure known as Texas Tower No. 4. Jacking devices somewhat similar to the cable jacks disclosed in the Nixon U.S. Pat. No. 2,833,118 were employed, but a great deal of time was wasted in threading the cables through the gripping devices before the hoisting operation could be started, and during which "set up" time the platform and caissons were in constant danger of being seriously damaged by wind and wave action. The danger continued during the jacking operation, which was very slow, until the platform was raised high enough to clear the waves.